The present invention relates to a process for the preparation of a powder in the form of grains, and which is particularly suitable for fritting, especially for producing large members having improved mechanical properties.
It is known to use alumina or alumina-based refractory materials e.g. in place of metal parts in mechanical assemblies operating at high temperatures, i.e. above 1000.degree. C., which is the case in heat motors. By comparison with other refractory ceramics, alumina or alumina-based composite materials have the advantage of a good thermal shock resistance. Moreover, their bending resistance is improved on carrying out a zirconia microdispersion in a finely divided alumina. This resistance rises from 200 to 350 MPa, when the crystallites have a size not exceeding a few microns.
The alumina or alumina-based composite materials are shaped in a conventional manner in accordance with the following industrial procedure:
1. preliminary treatment of the powder;
2. compression of a rough blank;
3. thermal treatment and natural fritting;
4. obtaining the required dimensions.
It is vital to obtain a rough compressed blank as close as possible to the final dimensions, if it is wished to industrially produce dense parts of large size and complicated geometry. It is therefore the capacity to produce such rough parts, which controls the practicability of the complete process. However, at present, rough blanks do not have an adequate strength to enable anything other than small simple objects to be produced on an industrial scale. Such a situation is very prejudicial to substituting metal parts operating at high temperatures by alumina-based ceramics, particularly in the case of large, complicated parts, despite the excellent performance levels which can be expected from alumina.
These difficulties are mainly due to the fact that the microcrystallization necessary for bringing about the mechanical reinforcement of the part can only be obtained by fritting the compressed object at a temperature equal to or below 1500.degree. C. This can only be obtained with agglomerated, extremely finely ground powders which, as a result of the grinding lack cohesion and are consequently unsuitable for agglomeration. The granulation of the powder by compression and crushing is not possible, as a result of the lack of intrinsic cohesion of granules.